Biasing drive torque to a secondary axle in a motor vehicle powertrain

ABSTRACT

A method for transmitting power to the wheels of a motor vehicle includes driveably connecting an output of a transmission to a first set of wheels, varying the magnitude of torque transmitted to an input of a differential mechanism, varying the speed of the input of the first differential mechanism, and driveably connecting the differential mechanism to the wheels of a second wheel set.

BACKGROUND OF THE INVENTION

This invention relates generally to a system and method for transmittingpower to the wheels of a motor vehicle.

Vehicle drive systems in the prior art employ an on/off braking device,which changes the transmission ratio between front driven axles and reardriven axles. Other systems use a mismatch in final drive ratios andcontrollable braking devices, i.e. two or more brakes, to vary themagnitude of torque transmitted to the driven axles.

In the prior art, the speed and torque transmitted to the left and rightrear wheels are controlled by a differential mechanism having twoslipping brakes (electromagnetic or hydraulic), one for the right axleshaft, the other brake for the left axle shaft.

U.S. Pat. No. 4,981,190 discloses a four wheel drive vehicle thatdirects torque to desired wheels by using a brake system. The brakesystem includes a brake control unit for controlling disk brakes. If oneof the wheels slips, the brake control unit stops the slip and sendstorque to other wheels.

U.S. Pat. No. 6,712,730 discloses a torque coupling that connects thesecondary driving wheels of an automotive vehicle indirectly to thepower unit of the vehicle through a variable torque coupling, while theprimary driving wheels on the vehicle are connected directly to thepower unit. The coupling compensates for variations in the angularvelocity of the primary and secondary wheels while still transferringtorque to the secondary wheels. The coupling includes a magneticparticle brake organized about an axis and a planetary gear setorganized about the same axis. The coupling has two paths through whichthe torque is transferred, one a mechanical path and the other a brakepath. Most of the torque is transferred through the mechanical path,while the brake path accommodates for slippage and controls theproportion of torque delivered to the secondary wheels, with the controlbeing solely dependent on the magnitude of the current directed throughthe brake.

It would be desirable to provide a system and method for varying themagnitude of torque transmitted to the input of a differential mechanismand adjusting the magnitude of differential torque applied to the leftand right rear wheels by controlling wheel brake torque at each of therear wheels in accordance with current vehicle handling requirements.

SUMMARY OF THE INVENTION

A system embodiment provides continuous, smooth variation of torquetransmission ratio in a single, compact unit. The drive system transmitstorque continuously to a first wheel set, preferably the front vehiclewheels, and a variable torque magnitude on-demand preferably to the rearwheels. The engine and transaxle preferably disposed transversely andlocated in the engine compartment at the front of the vehicle.

A method embodiment for transmitting power to the wheels of a motorvehicle includes driveably connecting an output of a transmission to afirst set of wheels, varying the magnitude of torque transmitted to aninput of a differential mechanism, varying the speed of the input of thefirst differential mechanism, and driveably connecting the differentialmechanism to the wheels of a second wheel set. The magnitude of drivetorque transmitted from the first differential mechanism differentiallyto the wheels of the second wheel set is changed by varying themagnitude of brake torque applied to a first wheel of the second wheelset relative to a magnitude of brake torque applied to a second wheel ofthe second wheel set.

The system includes a power transmitting device, located between thetransmission output and input of the rear differential, for changing thespeed and torque of a rear differential input. That device may include aplanetary gear unit, whose speed ratio and output torque can be variedby controlling a friction brake or brake. Alternatively, the device maybe a brake, whose output speed and torque vary with the degree of slipacross the brake. A slipping brake and planetary gear unit can be usedin combination to vary torque and speed to the rear differential input.

The scope of applicability of the present invention will become apparentfrom the following detailed description, claims, and drawings. It shouldbe understood, that the description and specific examples, althoughindicating preferred embodiments of the invention, are given by way ofillustration only. Various changes and modifications to the describedembodiments and examples within the spirit and scope of the inventionwill become apparent to those skilled in the art.

DESCRIPTION OF THE DRAWINGS

These and other advantages of the present invention will become readilyapparent to those skilled in the art from the following detaileddescription of a preferred embodiment when considered in the light ofthe accompanying drawings in which:

FIG. 1 is a schematic diagram of a powertrain for an automotive vehicle;and

FIG. 2 is schematic diagram of a speed multiplying gearset for use inthe powertrain of FIG. 1; and

FIG. 3 is schematic diagram of a speed multiplying gearset for use inthe powertrain of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first to FIG. 1, the powertrain for a motor vehicle 10includes front wheels 12, 14 and rear wheels 16, 18, each wheel fittedwith a tire. A power source 20, such as an internal combustion engine oran electric motor, is driveably connected to the input 21 of a transaxle22, which varies the speed and torque at the transmission output 24relative to the speed and torque at the output of the power source. Thetransmission 22 may be an automatic or manual transmission, whichproduces multiple discrete ratios of the speed of its output 24 dividedby the speed of its input 21, or a continuously variable transmission,such as a traction drive or belt drive transmission, which varies theoperating speed ratio steplessly.

The transmission output 24 is driveably connected directly to a frontdifferential mechanism 26, which transmits power through front axleshafts 28, 30 differentially to the left and right front wheels 12, 14.The rotational speed of the front wheels is controlled by front brakes32, 34, to which brake pressure from a source of pressurized brake fluid36, such a master cylinder, is supplied in response to the depressed andreleased state of a brake pedal 38, which is controlled manually by thevehicle operator.

The transmission output 24 is also driveably connected to a frontdriveshaft 40, which transmits power to a torque biasing device (TBD)42, whose output is driveably connected by a rear driveshaft 44 to arear differential mechanism 46. The rear wheels 16, 18 are driveablyconnected by rear axle shafts 48, 50 to the output of the reardifferential 46. The rear differential 46 may be a conventionalmechanism that transmits torque to the left and right rear wheels andaccommodates speed differential between the wheels. The rotational speedof the rear wheels 16, 18 is controlled by rear brakes 52, 54, to whichbrake pressure from the brake pressure source 36 is supplied in responseto the state of the brake pedal 38.

Under normal forward driving conditions when the front brakes areapplied, the wheel brake system delivers brake pressure that isapproximately equal side-to-side and produces brake forces applied tothe front left wheel F_(LF) and front right wheel F_(RF) that areapproximately mutually equal. Under normal forward driving conditions,when the rear brakes are applied, the wheel brake system delivers brakepressure that is approximately equal side-to-side and produces brakingforces applied to the rear left wheel F_(LR) and to the rear right wheelF_(RR) that are approximately mutually equal, but may be different inmagnitude from the front brake forces. The result is balancedtire-to-road forces that produce little or no yaw moment about thevehicle center of gravity.

The magnitude of net rear wheel torque at the left and right rear wheels16, 18, however, is also controlled differentially such that themagnitude of wheel brake torque applied at each rear wheel by thevehicle braking system under control of a brake control module (BCM) 56.The BCM 56 controls the differential application of brake pressure tothe rear brakes 52, 54 in accordance with determinations made duringexecution of a programmed control algorithm. The speed and torquetransmitted to the left rear wheel 16 by left rear axle shaft 48 isdetermined by the magnitude of pressure in the left rear wheel brake 52.Similarly, the speed and torque transmitted to the right rear wheel 18by right rear axle shaft 48 is determined by the magnitude of pressurein the right rear wheel brake 54.

Differential rear wheel brake torque is applied by the braking system inresponse to electronic command signals produced by a torque controlmodule (TCM) 60 to enhance vehicle stability and to augment vehiclehandling under various driving conditions. For example, over-steer andunder-steer conditions can be corrected by regulating net wheel torqueas described above. The TCM 60 communicates with both the BCM 56 and apowertrain control module (PCM) 62, which controls operation of theengine 20 and transmission 22.

The TBD 42 includes a coupler or brake 64, which includes an input 66,driveably connected to the front driveshaft 40, and an output 68,driveably connected to a gearset 70. The gearset's output is driveablyconnected to the rear driveshaft 44. The coupler/brake 64 alternatelydriveably connects and disconnects its input 66 and output 68, inresponse to signals transmitted by the TCM 60. When the coupler/brake 64is disengaged, the rear driveshaft 44 is disconnected from the frontdriveshaft 40, and there is no torque transfer through the coupler 64 orgearset 70. A residual drag torque across the coupler may be present;however, this residual torque would not be sufficient to drive thevehicle's wheels.

Preferably, gearset 70 is a planetary gearset, which increases the speedof the rear driveshaft 44 in relation to the speed of the front driveshaft 40. FIG. 2 illustrates a preferred gearset, which includes a sungear 72, a ring gear 74, a planet pinion carrier 76, and a set of planetpinions 78 supported on the carrier and engaged with the ring gear andsun gear. The carrier 76 is driveably connected to the output 68 ofcoupler 64, ring gear 74 is fixed against rotation, and sun gear 72 isdriveably connected to the rear driveshaft 44. When coupler 64 isengaged, sun gear 72 and driveshaft 44 are overdriven relative to thespeed of carrier 76 and driveshaft 44.

FIG. 3 illustrates another gearset embodiment, in which the sun gear 72is fixed against rotation, the ring gear 74 is driveably connected tothe rear driveshaft 44, and the carrier 76 driveably connected to theoutput 68 of coupler 64. When coupler 64 is engaged, ring gear 74 anddriveshaft 44 are overdriven relative to the speed of carrier 76 anddriveshaft 44.

In a positive torque condition, i.e., when power is transmitted from theengine to the wheels, the speed of front drive shaft 40 is determined bythe engine/transmission output 24. When the TBD 42 is active, i.e., whencoupler/brake 64 is engaged, the speed of rear driveshaft 44 is amultiple of the speed of front driveshaft 40, the multiple depending onthe speed ratio produced by gearset 70. When the TBD 42 is active,preferably the speed of driveshaft 44 is greater than the speed ofdriveshaft 40.

When the TBD 42 is inactive, i.e., when coupler/brake 64 is disengaged,the speed of driveshaft 44 is determined by the speed of the rear axles48, 50 and the drive ratio of the rear differential 46. The magnitude oftorque transmitted to driveshaft 44 from driveshaft 40 is determined bythe slip across TBD 42, which slip is a function of the magnitude ofpressure applied to actuate the coupler/brake 64.

The speed and torque transmitted to axleshafts 48, 46 is determined bythe magnitude brake torque in the respective wheel brakes 52, 54, whichbrake torque is a function of the magnitude of brake pressure applied tothe brakes.

The rear differential 46 may be a conventional mechanism that transmitstorque to the left and right wheels 16, 18 and accommodates speeddifferential between the wheels. However, the magnitude of net rearwheel torque at the left and right wheels will be controlled by varyingthe magnitude of wheel brake torque applied at each rear wheel by thevehicle braking system 56.

Differential rear wheel brake torque will be applied by the brakingsystem 56 in response to an electronic command signal produced by thepowertrain controller 62 as required to augment vehicle handling undercurrent driving conditions. For example, over-steer and under-steerconditions can be corrected by regulating net wheel torque as describedabove.

A brake control algorithm is preferably located in the BCM 56, but itcan be implemented in a central control module or any other module onthe vehicle multiplex bus. The BCM 56 or another relevant modulereceives the following input signals produced from various sensors:engine speed, engine torque, throttle position, actual torquetransferred, maximum possible torque transfer, speed of each wheel 12,14, 16, 18, vehicle speed, yaw rate, lateral and longitudinalacceleration of the vehicle, and steering wheel angle. The brake controlalgorithm uses these inputs to calculate the desired torque at eitherthe left or right rear wheel. To achieve the desired torque, the brakecontrol algorithm produces the following command signals: commandedtorque transfer, brake pressure to be supplied to the left rear brake52, and brake pressure to be supplied to the right rear brake 54.

In accordance with the provisions of the patent statutes, the presentinvention has been described in what is considered to represent itspreferred embodiment. However, it should be noted that the invention canbe practiced otherwise than as specifically illustrated and describedwithout departing from its spirit or scope.

1. A method for transmitting rotating power to the wheels of a motorvehicle comprising the steps of: (a) driveably connecting an output of atransmission to a first set of wheels; (b) varying the magnitude oftorque transmitted to an input of a first differential mechanism; (c)varying the speed of the input of the first differential mechanism; and(d) driveably connecting the first differential mechanism to the wheelsof a second wheel set.
 2. The method of claim 1 further comprising: (e)changing the magnitude of drive torque transmitted from the firstdifferential mechanism differentially to the wheels of the second wheelset by changing a magnitude of brake torque applied to a first wheel ofthe second wheel set relative to a magnitude of brake torque applied toa second wheel of the second wheel set.
 3. The method of claim 1 whereinstep (a) further comprises: driveably connecting the output of thetransmission to an input of a second differential mechanism; anddriveably connecting the second differential mechanism to the wheels ofthe first wheel set.
 4. The method of claim 1 wherein step (c) furthercomprises: operating a gearset located in a torque delivery path betweenthe output of the transmission and the input of the first differentialmechanism such that the speed of the input of the first differentialmechanism is greater than the speed of the output of the transmission.5. The method of claim 1 wherein wherein step (c) further comprises:operating a gearset located in a torque delivery path between the outputof the transmission and the input of the first differential mechanismsuch that the speed of the input of the first differential mechanism isgreater than the speed of the output of the transmission; and step (b)further comprises: locating a coupler in a torque delivery path betweenthe output of the transmission and the gearset; and changing the torquetransmitting capacity of the coupler such that the magnitude of torquetransmitted to an input of the gearset changes.
 6. A method fortransmitting power to the wheels of a motor vehicle comprising the stepsof: (a) driveably connecting an output of a transmission to firstdifferential mechanism; (b) driveably connecting an output of the firstdifferential mechanism to a first set of wheels; (c) driveablyconnecting the output of the transmission to a gearset; (d) using thegearset to increase the speed of the output of the transmission; (e)driveably connecting an output of the gearset to an input of a seconddifferential mechanism; and (f) driveably connecting the seconddifferential mechanism to the wheels of a second wheel set.
 7. Themethod of claim 6 further comprising: (g) changing the magnitude ofdrive torque transmitted from the second differential mechanismdifferentially to the wheels of the second wheel set by changing amagnitude of brake torque applied to a first wheel of the second wheelset relative to a magnitude of brake torque applied to a second wheel ofthe second wheel set.
 8. The method of claim 1 wherein step (d) furthercomprises: locating a coupler in a torque delivery path between theoutput of the transmission and the gearset; and changing the torquetransmitting capacity of the coupler such that the magnitude of torquetransmitted to an input of the gearset changes.
 9. A powertrain for anautomotive vehicle having first and second sets of wheels comprising: apower source; a transmission including a first input driveably connectedto the power source and a first output driveably connected to the wheelsof the first wheel set; a torque biasing device including a second inputand a second output, driveably connected to the wheels of a second wheelset, for increasing a speed of the second output relative to a speed ofthe second input; and a wheel brake system for changing a magnitude ofbrake torque applied to a first wheel of the second wheel set relativeto a magnitude of brake torque applied to a second wheel of the secondsaid wheel set.
 10. The powertrain of claim 9, further comprising: afirst differential mechanism driveably connected to the first output andthe first set of wheels for driving the wheels of the first wheel set.11. The powertrain of claim 9, further comprising: a second differentialmechanism driveably connected to the second output and the second wheelset, for driving the wheels of the second wheel set.
 12. The powertrainof claim 9, wherein the first wheel of the second wheel set is locatedat a first lateral side of the vehicle; and the second wheel of thesecond wheel set is located at a second lateral side of the vehicleopposite the first side.
 13. The powertrain of claim 9 furthercomprising: a coupling driveably connected to the first output andhaving a variable torque transmitting capacity for driveably connectingthe first output and the second input.